1. Field of the Invention
The present invention relates to a technology for protecting a memory from an unauthorized access.
2. Description of the Related Art
Computers take advantage of various resources such as a memory and a central processing unit (CPU) by using a multiprogramming system, which performs a plurality of processes such as programs, tasks, and jobs by switching among them.
Such a computer involves a risk that data in the memory used by a certain process can be accessed or overwritten by another process and damaged.
To protect the data from such a risk, z/Architecture Principles of Operation (IBM; SA22-7832-00; December 2000; pp. 3-9 to 3-12) discloses a typical technology of determining accessibility by prestoring a storage key with respect to each of predetermined size of memory region such as four kilobytes, and checking the storage key of a process with that of the memory to be accessed by the process.
However, a memory protection method that uses the storage keys requires a memory that stores therein the storage keys in addition to the memory that stores therein the data. Thus, the amount of hardware increases.
Moreover, because the data is protected by the unit of four-kilobyte page, protection of small data wastes some memory area that remains unused. Although the problem can be solved by assigning the storage keys by the smaller unit, the solution will require more memory to store the storage keys.